1. Field of the Invention
The present invention relates to a distortion correction device that corrects an image signal obtained by image sensing for distortion produced by the lens used for the image sensing, and to an image sensing device provided with such a distortion correction device
2. Description of Related Art
When an image is sensed with a solid-state image sensing device such as a CCD (charge-coupled device) or CMOS sensor through an optical lens, the aberrations produced by the optical lens cause the imaging positions on the solid-state image sensor to be displaced. Thus, if the image is reproduced for display directly from the image signal outputted from the solid-state image sensing device, the displayed image appears deformed. This deformation appearing in the displayed image is called distortion. Different types of distortion appear depending on the design of the optical lens and on the arrangement of the optical lens and the solid-state image sensing device relative to each other. One typical type of distortion is “pincushion-shaped distortion” in which the imaging positions are stretched compared with the ideal imaging positions as shown in FIG. 17A, and another is “barrel-shaped distortion” in which the imaging positions are shrunk compared with the ideal imaging positions as shown in FIG. 17B. In FIGS. 17A and 17B, dotted lines indicate the ideal image positions.
The value of distortion that appears as shown FIG. 17A or 17B varies non-linearly with respect to the image height, i.e., the distance from the optical axis of the optical lens to a given imaging position (corresponding to a given pixel position on the solid-state image sensing device). In one conventionally proposed image sensing device, the values of distortion corresponding to different pixel positions are stored in a data table, which is later referred to so that the distortion of the image is corrected for based on the values, stored in the data table, of distortion corresponding to different pixel positions. Disadvantageously, however, this image sensing device requires a large-capacity memory for the storage of distortion values corresponding to different pixel positions.
To overcome this disadvantage, in another conventionally proposed image sensing device, distortion (the distortion factor) D is approximated as proportional to the square of the image height, and a calculation circuit is provided that corrects for optical distortion based on distortion factors thus approximated. This eliminates the need for a memory for storing correction values corresponding to different pixel positions (see Japanese Patent Application Laid-Open No. H6-153065). In another conventionally proposed image sensing device, distortion values of an image is corrected with respect to an arbitrary image height so that distortion is corrected less in the negative direction. This, compared with when distortion is corrected with respect to the center of the image corresponding to the optical axis, permits the image to fill a wider area on the image sensing surface (see Japanese Patent Application Laid-Open No. H6-165024). In still another conventionally proposed image sensing device, through calculation performed based on a quadratic approximation polynomial by an approximation polynomial calculator, pixel positions before distortion correction corresponding to pixel positions after distortion correction are calculated, and interpolation calculation is performed differently for the brightness and color-difference signals corresponding to the thus calculated pixel positions before distortion correction (see Japanese Patent Application Laid-Open No. H11-250239).
With the image sensing device proposed in Japanese Patent Application Laid-Open No. H6-153065 mentioned above, simply providing the calculation circuit makes it possible to correct for distortion based on distortion values without using a memory for storing correction values corresponding to different pixel positions. This, however, cannot deal with all types of distortion, because not all distortion is proportional to the square of the image height. With the image sensing device proposed in Japanese Patent Application Laid-Open No. H6-165024 mentioned above, distortion is evaluated with respect to a circle rather than the center corresponding to the optical axis, and this permits the image sensing surface of the image sensing device to be used as efficiently as possible. Here, however, the approximation formula is determined according to the zoom position, and either the approximation formula itself or the correction values corresponding to different pixel positions as calculated based on it are stored in a memory. Thus, when correction values corresponding to different pixel positions are stored, a large-capacity memory is needed.
With the image sensing device proposed in Japanese Patent Application Laid-Open No. H 11-250239 mentioned above, for signals after distortion correction using the approximation formula, brightness signals, to which the sensitivity of the human eye is high, are calculated by an interpolation formula that requires much calculation, and color-difference signals, to which the sensitivity of the human eye is low, are calculated by an interpolation formula that requires less calculation. Thus, even when a lens that produces large distortion is used, calculation can be performed quickly. However, for the approximation formula used for distortion correction of coordinate positions, the coefficients for the approximation formula as determined according to the focal length are stored. Thus, when the actual focal length differs from the set focal length, the coefficients for the approximation formula cannot be calculated.